1. Field of the Invention
This invention relates to a gamma-ray compensated neutron ionization chamber for measuring only the neutron flux in the presence of gamma-rays in a nuclear reactor or the like.
2. Description of the Prior Art
FIG. 7 shows a gamma-ray compensated neutron ionization chamber in which holes are formed in a signal electrode as disclosed in Japanese Laid-open Patent Application No. Hei 4-363856. In FIG. 7, reference numeral 1 denotes a cylindrical high-voltage electrode, 2 a cylindrical signal electrode arranged concentrically within the high-voltage electrode with predetermined spacing therebetween, 3 a cylindrical compensating electrode arranged concentrically within the signal electrode 2 with predetermined spacing therebetween, 4 a high-voltage power source for applying a high voltage +VH to the high-voltage electrode 1, 5 a compensating power source for applying a compensating voltage -VC to the compensating electrode 3, 6 an amplifier for amplifying a neutron current obtained from the signal electrode 2, 7 a neutron sensitive substance such as 10.sub.B, coated on the inner surface (surface facing the signal electrode 2) of the high-voltage electrode 1, 8 a neutron sensitive substance such as 10.sub.B coated on the outer surface(surface facing the high-voltage electrode 1) of the signal electrode 2, 10 a neutron ionization chamber which is ionization space between the high-voltage electrode 1 and the signal electrode 2, 11 a compensated ionization chamber which is ionization space between the signal electrode 2 and the compensating electrode 3, and 12 a plurality of holes formed in the signal electrode 2. These holes 12 are arranged in peripheral and axial directions of the signal electrode 2 as shown in FIG. 8.
The above publication discloses an example in which 48 holes 12 having a diameter of 5 mm are formed but does not mention limitations on the diameter of each hole, the number of holes and the distance between hole axes (distance between adjacent holes). However, as the volume of a leakage electric field produced by each of the holes 12 correspond to the width of gamma-ray compensation, the volume of the leakage electric field is desirably controlled to a value equal to or smaller than several percents of the volume of the neutron ionization chamber 10. This is because a change in signal current at a voltage near use voltage is desirably as small as possible when neutrons and gamma-rays remain unchanged and must be 5%/100 V (this means that a signal change is 5% or less when the use voltage is changed by 100 V). When the area of the hole 12 is large, a change in signal current is large and the gamma-ray compensated neutron ionization chamber cannot be used. The saturation characteristics of the neutron ionization chamber 10 will be described with reference to FIG. 2. In FIG. 2, P shows when there are no holes 12 in the signal electrode 2, Q shows when the total area of the holes 12 in the signal electrode 2 is 5% of the surface area of the signal electrode 2, and R shows when the total area of the holes 12 in the signal electrode 2 is 10% of the surface area of the signal electrode 2. It is understood from FIG. 2 that when the total area of the holes 12 is larger than 5% of the surface area of the signal electrode 2, deterioration in saturation characteristics becomes marked, which is a problem in practical application. The surface area of the signal electrode 2 is the surface area on either one of a high-voltage electrode 1 side and a compensating electrode 3 side of the signal electrode 2.
In the gamma-ray compensated neutron ionization chamber of the prior art, saturation characteristics deteriorate in proportion to the total area of the holes 12 because the neutron ionization chamber 10 and the compensated ionization chamber 11 are not sealed hermetically as described above. When the volume of a leakage electric field produced by each of the holes 12 increases, the width of gamma-ray compensation expands with the result of a growing signal change (signal error).
As shown in FIG. 9, when the interval W2 between hole axes which is the distance between adjacent holes 12 is set to a value smaller than the interval between the signal electrode 2 and the compensating electrode 3, that is, the thickness D of the compensated ionization chamber 11 (W2&lt;D), the leakage electric fields E2 of the holes 12 form a continuous waveform distribution. The volume of the leakage electric field is larger than that when the effect of each of the holes 12 is evaluated independently, thereby causing such a problem as a growing signal change.
As shown in FIG. 10, a hole 13 corresponding to the hole 12 is formed tapered. That is, when the hole is formed such that its diameter becomes smaller towards the compensating electrode 3 side from the high-voltage electrode 1 side, a portion 14a where the wall surface 13a of the hole intersects the outer surface (surface facing the signal electrode 1) of the signal electrode 2 and a portion 14b where the wall surface 13a of the hole intersects the inner surface 2b (surface facing the compensating electrode 3) of the signal electrode 2 form an edge. Particularly, the portion 14b where an angle formed by the wall surface 13a and the inner surface 2b is acute forms a shape edge. In this case, as shown in FIG. 11, the concentration of fields E3 readily occurs in the sharp edged portion 14b, and changes in leakage electric field produced by each of the holes 13 are sharp and readily affected by changes in compensating voltage. In addition, to evaluate the effect of each of the holes 13 independently, the interval W2 between hole axes shown in FIG. 9 must be made large, thereby making more strict limitations on the interval W2 of hole axes.